The research tests the hypothesis that coronary flow reserve as measured by the reactive hyperemic response of coronary arteries in humans contains important information about the physiologic significance of epicardial coronary arterial lesions which could influence patient selection for revascularization procedures and patient management. This project has eight specific aims: (1) to develop and validate a thermistor flow probe for safely determining coronary blood flow velocity in patients during routine cardiac catheterization. This will involve in vitro as well as in vivo canine experiments, (2) to use the thermistor flow probe to develop and validate an easy to use anemometric method for determining exactly coronary lesion percent stenosis, (3) to establish the range of normal human coronary blood flow velocity and coronary vasodilator reserve capacity using the thermistor flow probe, (4) to correlate coronary flow reserve with several quantitative angiographic lesion-associated variables, (5) to document the changes in coronary flow reserve immediately after percutaneous transluminal coronary angioplasty (PTCA), (6) to correlate coronary flow reserve with exercise left ventricular (LV) wall motion abnormalities using radionuclide ventriculography, (7) to evaluate the no-reflow phenomenon by examining the effect of prolonged myocardial ischemia on coronary flow reserve measured immediately, and 3 months following acute reperfusion of patients with acute myocardial infarction, (8) to determine the rapidity with which coronary collateral channels begin to function during brief coronary artery occulsions in man.